Method of coating steel electrostatically with aluminum powder coated with a higher fatty-acid salt

ABSTRACT

A STEEL SURFACE IS COATED ELECTROSTATICALLY WITH ALUMINUM POWDER WHICH IS COATED WITH A SODIUM, CALCIUM OR ZINC SALT OF A SATURATED HIGHER FATTY ACID HAVING FROM ABOUT 16 TO 18 CARBON ATOMS. THE COATED STEEL SURFACE IS THEN HEATED, ROLLED AND HEATED.

1972 HIDEHISA YAMAGISHI EI'AL 3,700,477

' METHOD OF COATING STEEL ELECTROSTATIGALLY WITH ALUMINUM POWDER COATED WITH A HIGHER FATTY-ACID SALT Filed Oct. 7, 1970 2 Sheets-Sheet 2 FIG. 5

FIG. 5

United States Patent a 3 700 477 I METHOD OF coATn ro ,STEEL ELECTROSTA CALLY WITH ALUMINUM POWDER COATED WITH A HIGHER FATTY-ACID SALT Hidehisa Yamagishi, Fumitoshi. Yokoi, and Tuyoshi Kutino, all Nippon Kokan Kabushiki Kaisha Technical Institute, 2730 Minamiwataridachio, Kawasakaishi, Kawasakai, Japan 1 Continuation-impart of abandoned application SenNo.

. 720,178, Apr. 10,1968. This application 0ct...7, 1.970,

: SenNo. 78,754

Claims priority, application Japan, Apr. '13, .1967,

42/23,315; Great Britain, Apr. 10, 1968, 17,304/6 8 Int; Cl. B4411 1/094; C23c 17/02 I US. Cl. 117-17 7 ABSTRACT OF THE DISCLOSURE salt of a saturated higher fatty acid having from about 16 to 18 carbon atoms. The coated steel surface is then heated, rolled and heated.

RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 720,178, filed Apr. 10, 1968 which has been abandoned.

This invention relates to a method of electrostatic coating steel or other metals with aluminum powder, characterized by a superior aluminum coated layer with a high lustre and good adherence.

In the prior art, a method of coating the steel surface with aluminum powder is disclosed in Patent Publication No. 640,152 of the Union of South Africa, but because the aluminum powder covered only with aluminum oxide film is short of electric insulation for electrostatic coating, said aluminum powder adhered on the steel plate is electrically discharged and easily dislodged from the steel surface due to the vibration and impact during the subsequent treatment stages. As the conventional powder is comparatively lacking in electric charge, high speed operation is impossible. Further, if the steel plate is rolled after drying at low temperature to the extent that the water on the surface is removed as proposed in said patent publication, the aluminum powder is inclined to adhere on the surface rolls, which not only lowers the yield, but also produces grained or feathered patterns on the surface. Accordingly, beautiful surface characteristics With a high lustre are not available and, moreover, the above described prior method requires special treatment at high temperature to improve the adherence, which makes the treatment time prolonged so as to be undesirable for continuous operation.

Accordingly, it is the object of this invention to provide a novel method of electrostatic coating of a steel surface with aluminum powder including the steps of preventing said aluminum powder from dislodging from the steel surface by coating the outside of an aluminum oxide film with a small amount of higher fatty acid salt to increase the coating efficiency of said aluminum powder on the steel surface during electrostatic coating, thereby improving the electrical insulation and thereafter of holding the treated steel plate in a reducing inert or oxidizing atmosphere at 400 C.680 C. for l80 seconds to avoid the adherence of said aluminum powder to rolls during rolling operation and to obtain a beautiful and lustrous surface after rolling. If the atmospheric temperature is 400 C. or less, it is necessary for the steel plate to be held for prolonged time interval which naturally makes high speed 1 Patented Oct. 24, 1972 be obtained if a steel plate of more than 0.2 mm. thick is held at- 400" C.-680 C. for 5-180 seconds. After rolling, the-heat treatment may be applied when it is necessary toimprove the adherence of aluminum powder to the steel plate. In this case, the heat treatment time is less than 20 minutes at 500 C. if pre-heating is applied, almost less than one third compared with the case of no pre-heating.

The'inventionis illustrated in the accompanying drawings in-whichz,

FIG. 1 shows a diagram of a manufacturing process according to the present invention.

FIG. 2 is a; graph indicating the optimum pre-heating range.

FIG. 3 is a photograph showing the surface characteristics of an aluminum-coated steel plate obtained in the examples. FIG."4 is a photograph showing the surface characteristics of an aluminum-coated steel plate obtained without pre-heating. FIG. 5 is a photograph showing the surface characteristics of an aluminum-coated steel plate of Comparative Example A.

As shown in FIG. 1, the manufacturing process of the present invention is continuously integrated, that is, the steel strip fed from uncoiler 1 is successively treated in electrolytic cleaning tank 3, washing tank 4, electrolytic pickling tank 5, Washing tank 6 and scrubber 7, and then dried in dryer 8; thereafter the strip, coated at the electrostatic coating stage 9, is heated in pre-heating furnace 10 at 400 C.-680 C. in a reducing, inert or oxidizing atmosphere, rolled by rolling mill 11 and finally, coiled by recoiler 13 after heat treatment is finished in heat treatment furnace 12. FIG. 3 shows that in the present invention characterized by powder coating of a higher fattyacid salt, uniform and beautiful surfaces with a high lustre can be obtained, while in said South- African Patent the surface shows an uneven pattern when no special insulation layer is provided. Similarly, as clearly shown in FIGS. 3 and 4, a comparison of the steel plate of the present invention, applying a short pre-heating treatment at high temperature, with one of the conventional method, shows a marked improvement in the surface characteristics. According to the experiment to study the optimum pre-heating condition for steel plates of 0.8 mm. thick, it is proved preferable as shown in FIG. 2 that the steel plate is preheated at 400 C.-680 C. for 5-180 seconds.

The invention is illustrated further by the following examples.

EXAMPLE 1 (A) Steel plate: rimmed steel strip (0.8 mm. thick and 300 mm. wide) (B) Aluminum powder: particle size: 325 mesh. The powder was coated with 0.2 wt. percent sodium stearate (C) Electrostatic coating: voltage: 90,000 v. powder carrying gas: N coating weights: 70 g./m.

(D) Preheating: heating for 25 seconds in an oxidizing atmosphere (air) at 650 C.

'(E) Rolling: rolling speed: 40 m./min. rolling load:

ton per 300 mm. width (F) Heat treatment: heating for 20 minutes in the oxidizing atmosphere at 500 C.

The surface of aluminum-coated steel plates obtained under the above-mentioned conditions was beautiful with lustre which was characteristic of aluminum and superior in adhesiveness, corrosion resistant property and heat 3 resistant property, and showed no defects as the result of a deep drawing test. Moreover, as the result of salt spray test (ASTM-B117-62 1965), red rust was not formed for more than 2,000 hours.

EXAMPLE 2 (A) Steel plate: rimmed steel strip (0.25 mm. thick and 300 mm. wide) (B) Aluminum powder: particle size: 325 mesh. The powder was coated with 0.3 wt. percent calcium stearate (C) Electrostatic coating same as in Example 1 (D) Preheating: heating for 10 seconds in an oxidized atmosphere (air) at 650 C.

(E) Rolling: same as in Example 1.

Aluminum-coated steel plate obtained under the abovementioned conditions has excellent adhesiveness and excellent corrosion resistant properties. When left in an oxidized atmosphere (air) at 750 C. for 250 hours, the aluminum-coated steel plate weight gains less than 3 mg./cm. due to oxidation and showed excellent heat resistant propetry equivalent to 13 Cr steel.

EXAMPLE 3 The aluminum-coated steel plate obtained under the above-mentioned conditions has the same excellent properties as those of Example 1 and 2.

EXAMPLE 4 (A) Steel plate: rimmed steel strip (0.5 mm. thick and 200 mm. wide) (B) Aluminum powder: same as Example 1 (C) Electrostatic coating: same as Example 1 (D) Preheating: heating for 180 seconds in a reducing atmosphere (H 20%, N 80%) at 450 C.

(E) Rolling: same as Example 3 (F) Heat treatment: same as Example 3.

The aluminum-coated steel plate obtained under the above-mentioned conditions was of the same excellence as that of Examples 1-3 in its formability, corrosion resistant property and heat resistant property.

EXAMPLE 5 (A) Steel plate: rimmed steel strip (0.8 mm. thick and 300 mm. wide) (B) Aluminum powder: same as Example 2 (C) Electrostatic coating: same as Example 1 (D) Preheating: heating for 25 seconds in a reducing atmosphere (decomposed ammonia gas) (E) Rolling: same as Example 1 (F) Heat treatment: same as Example 1.

The aluminum-coated steel plate obtained under the above-mentioned conditions was as excellent as that of Example 1-4.

EXAMPLE 6 (A) Steel plate: rimmed steel strip (0.25 mm. thick and 300 mm. wide) (B) Aluminum powder: same as Example 3 (C) Electrostatic coating: same as Example 1 (D) Preheating: heating for 30 seconds in an inert atmosphere (N at 650 C.

(E) Rolling: same as Example 1 (F) Heat treatment: same as Example 1.

4 The aluminum-coated steel plate treated under the above-mentioned conditions was as excellent as that of Examples 1-5.

COMPARATIVE EXAMPLE A (A) Steel plate: rimmed steel strip (0.8 mm. thick and 300 mm. wide) (B) Aluminum powder: powder of 325 mesh not coated with stearate (C) Electrostatic coating: same as Example 1 (D) Preheating: heating for 25 seconds in an oxidizing atmosphere (air) at 650 C.

(E) Rolling: same as Example 1 (F) Heating: same as Example 1.

The aluminum layer after being subjected to rolling was feather-like because the aluminum layer on the surface of the steel plate exfoliated partly due to the vibration after the electrostatic coating. The corrosion resistant property and heat resistant property thereof were unsatisfactory. 41

The following was proved by the results of the embodiments described hcreinbefore.

Aluminum powder obtains excellent properties by electrostatic coating after coated with from about 0.2 to about 0.4 wt. percent sodium stearate, calcium stearate or zinc stearate, or of such a metal salt of a saturated fatty acid having from about 16 to about 18 carbon atoms per molecule.

Preheating is carried out in a reducing or oxidizing atmosphere at 450650 C.; at 650 C. for a relatively short time as about 10 to about 30 seconds and at 450 C. for a relatively long time, as up to about 3 minutes. A reducing atmosphere is preferable, but the operation can be completed in an oxidizing atmosphere such as air which is not a vigorous oxidizing atmosphere.

It is apparent, therefore, that with the present method characterized by electrostatic coating steel material with aluminum or other metal powder of 325 mesh or less coated with a higher fatty-acid salt, it is possible to prevent said powder from falling off from the steel surface during treatment because of improved insulation and also to increase the electrostatic charge of said powder for its fineness to be suitable for high-speed operation. The powder is applied to the steel in the dry state. In the present method, a steel plate is rolled after pre-heating treatment for 5-180 seconds within the temperature range of 400 C.680 C., it is possible to effectively and properly form a beautiful and uniform aluminum-coated layer with a high lustre and good adherence. Moreover, the aluminum powder according to the present invention can be used as a lubricant for steel plates, tubes and bars during rolling and not only aluminum but also zinc, lead, tin and nickel powders coated with a higher fatty-acid salt can be applied to various surfaces under the properly chosen pre-heating conditions.

While the invention has been explained by describing a particular embodiment thereof, it will be apparent that improvements and modifications may be made without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. In a method for coating a steel surface electrostatically with a metal, wherein the metal is applied to said surface during an electrostatic deposition step and the coated surface is then subjected to heating in a reducing, inert or oxidizing atmosphere of from about 450 C. to about 650 C. for from 3 minutes to about 10 seconds, and is then rolled and heated, the improvement which comprises so applying aluminum metal in powder form coated with a sodium, calcium or zinc salt of a saturated higher fatty acid having from about 16 to about 18 carbon atoms.

2. The method of claim 1, wherein the metal has a particle size up to 325 mesh.

3. The method of claim 1, wherein the acid is stearic acid.

4. The method of claim 1, wherein the salt is sodium stearate.

5. The method of claim 1, wherein the salt is calcium 5 stear ate.

6. The method of claim 1, wherein the salt is zinc stearate.

7. The method of claim 1, wherein the amount of the salt is from about 0.2 to about 0.4 percent by weight 10 of the weight of the metal.

6 4/1952 Edwards 117-22 3/ 1960 Stetson 117-22 10/1960 McMillan et a1. 117-100 6/1961 Toulmin 11717 7/1961 Tarpley et a1. 117100 2/ 1962 Clayton 117100 5/ 1968 Wren et a1. 117-17 2/1969 Shimose et a1. 11722 3/1970 Austin 117131 2/ 1971 Costelloe et a1. 117131 FOREIGN PATENTS 1/ 1936 Great Britain 117--22 WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R.

117-22, 27, 31, 65.2, 46 CA, 130 R, 131, 93.4 R;

McGraw et a1. 111-22 15 Pfeil 117 -22 Pfeil 117 22 Nuesch et a1 117100 Grinlington 117--22 Bain 106-49 72 47 

